Cryogenic Refrigeration Method And Device

ABSTRACT

The invention relates to a cryogenic refrigeration device intended to transfer heat from a cold source to a hot source via a working fluid flowing through a closed working circuit including the following portions in series, namely: a portion for the substantially isothermal compression of the fluid, a portion for the substantially isobaric cooling of the fluid, a portion for the substantially isothermal expansion of the fluid, and a portion for the substantially isobaric heating of the fluid. The compression portion of the working circuit includes at least two compressors disposed in series and the expansion portion of the working circuit includes at least one expansion turbine, said compressors and expansion turbine(s) being driven by at least one high-speed motor including an output shaft. One end of the output shaft supports and rotates, by means of direct coupling, a first compressor, while the other end of the output shaft supports and rotates, by means of direct coupling, a second compressor or an expansion turbine.

The present invention relates to a cryogenic refrigeration device and method.

The invention relates more particularly to a cryogenic refrigeration device for transferring heat from a cold source to a hot source via a working fluid flowing in a closed working circuit, the working circuit comprising in series: a compression portion, a cooling portion, an expansion portion and a heating portion.

The cold source may for example be liquid nitrogen for cooling and the hot source water or air.

Refrigerators known for cooling superconductor elements generally use a reverse Brayton cycle. These known refrigerators use a lubricated rotary screw compressor, a countercurrent plate heat exchanger and an expansion turbine.

These known refrigerators have many drawbacks including:

-   -   low energy efficiency of the cycle and hence of the         refrigerator,     -   the use of oil to cool and lubricate the compressor, which         imposes a de-oiling of the working gas after compression,     -   the use of rotary seals between the electric motor and the         compressor,     -   the low isothermal compression efficiency of the compressor,     -   the frequency of maintenance operations.

Document U.S. Pat. No. 3,494,145 describes a refrigeration system using couplings via gears requiring oil-lubricated bearings. This type of device uses rotary seals such as mechanical seals between the working gas and the gear housing and oil bearings. This architecture increases the risks of leakage of the working gas and the potential pollution of the working gas by the oil. This system is also associated with a low-speed motor.

Document U.S. Pat. No. 4,984,432 describes a refrigeration system using compressors or liquid seal turbines operating with a low-speed motor using conventional bearings such as ball bearings. This technology is associated with positive displacement compressors and turbines.

It is an object of the present invention to overcome all or some of the drawbacks of the prior art identified above.

For this purpose, the invention proposes a cryogenic refrigeration device for transferring heat from a cold source to a hot source via a working fluid flowing through a closed working circuit, the working circuit comprising in series: a portion for the substantially isothermal compression of the fluid, a portion for the substantially isobaric cooling of the fluid, a portion for the substantially isothermal expansion of the fluid, and a portion for the substantially isobaric heating of the fluid, the compression portion of the working circuit comprising at least two compressors disposed in series and at least one heat exchanger for cooling the compressed fluid disposed at the outlet of each compressor, the expansion portion of the working circuit comprising at least one expansion turbine and at least one heat exchanger for heating the expanded fluid, the compressors and the expansion turbine(s) being driven by at least one high-speed motor comprising an output shaft whereof one end supports and rotates, by means of direct coupling, a first compressor and whereof the other end supports and rotates, by means of direct coupling, a second compressor or an expansion turbine.

The embodiments serve to obtain a system without oil pollution and without contact. This is because the combination of centrifugal compressors, centripetal turbines and bearings according to the invention reduces or eliminates any contact with the fixed parts and the rotating parts. This serves to avoid any risk of leakage. The overall system is in fact hermetically sealed and does not comprise any rotary seal with regard to the atmosphere (such as mechanical seals or dry face seals).

Moreover, embodiments of the invention may comprise one or more of the following features:

-   -   the compressors are of the centrifugal compression type,     -   the expansion turbine(s) are of the centripetal expansion type,     -   the output shafts of the motors are mounted on magnetic bearings         or on dynamic gas bearings, said bearings being used to support         the compressors and the turbines,     -   the cooling portion and the heating portion comprise a common         heat exchanger through which the working fluid flows in         countercurrent according to whether it is cooled or heated,     -   the working circuit comprises a volume forming a buffer storage         chamber for the working fluid,     -   the working fluid is in the gas phase and is composed of a pure         gas or a mixture of pure gases selected from: helium, neon,         nitrogen, oxygen, argon, carbon monoxide, methane, or any other         fluid having a gas phase at the temperature of the cold source.

The invention further proposes a cryogenic refrigeration method for transferring heat from a cold source to a hot source via a working fluid flowing through a closed working circuit, the working circuit comprising in series: a compression portion comprising at least two compressors disposed in series, a fluid cooling portion, an expansion portion comprising at least one expansion turbine, and a heating portion, the method comprising a working cycle comprising a first step of substantially isothermal compression of the fluid in the compression portion by cooling the compressed fluid at the outlet of the compressors, a second step of substantially isobaric cooling of the fluid in the cooling portion, a third step of substantially isothermal expansion of the fluid in the expansion portion by heating the expanded fluid at the turbine outlet, and a fourth step of substantially isobaric heating of the fluid having exchanged heat with the cold source, the fluid working cycle (temperature T, entropy S) being of the reverse Ericsson type.

Furthermore, embodiments of the invention may comprise one or more of the following features:

-   -   during the first substantially isothermal compression step, the         compressed fluid is cooled at the outlet of each compressor to         keep the fluid temperatures at the inlet and outlet of each         compressor substantially equal and preferably within a range of         about 10 K,     -   during the third substantially isothermal expansion step the         expanded fluid is cooled at the outlet of each turbine to keep         the fluid temperatures at the inlet and outlet of each turbine         substantially equal and preferably within a range of about 5 K,     -   the compressors and the expansion turbine(s) are driven by at         least one high-speed motor comprising an output shaft whereof         one end supports and rotates, by means of direct coupling, a         first compressor and whereof the other end supports and rotates,         by means of direct coupling, a second compressor or an expansion         turbine, and in that the method comprises a step of transfer of         part of the mechanical work of the turbine(s) to the         compressor(s) via the output shaft(s),     -   on completion of the second cooling step, the working fluid is         cooled to a low temperature of about 60 K and in that the         working circuit comprises a number of compressors that is about         three times higher than the number of expansion turbines,     -   the working fluid is used to cool or to keep cold superconductor         elements at a temperature of about 65 K,     -   the temperature drop of the fluid constituting the cold source         is substantially identical to the temperature rise of the         working gas in the heat exchangers.

The invention may have one or more of the following advantages:

-   -   the working fluid cycle (reverse Ericsson type) serves to obtain         a higher efficiency than the known systems but without         necessarily creating or increasing other drawbacks,     -   the expansion work in the turbines can be advantageously         utilized,     -   it is possible to eliminate the use of oil for lubrication or         cooling, so as to eliminate the de-oiling installation         downstream of the compressor, and palso the spent oil treatment         and recycling operations,     -   the system only requires a small number of moving parts, thereby         increasing its simplicity and reliability. Thanks to the         invention, it is possible, for the compressor, to do without a         mechanical power transmission of the type with speed step-up         gear or Cardan joints, etc.,     -   maintenance operations are reduced or even virtually         nonexistent,     -   the system serves to avoid rotary seals and to use a completely         hermetically sealed system with regard to the exterior. This         prevents any loss or pollution of the working cycle gas,     -   the size of the refrigerator may be reduced in comparison with         known systems.

Other features and advantages will appear from a reading of the description below, provided in conjunction with the figures in which:

FIG. 1 is a schematic view showing the structure and operation of a first exemplary embodiment of the refrigeration device according to the invention,

FIG. 2 schematically shows a detail of FIG. 1 showing an arrangement of a drive motor of a compressor-compressor or compressor-turbine assembly,

FIG. 3 schematically shows an example of a working cycle of the working fluid of the refrigerator in FIG. 1,

FIG. 4 is a schematic view showing the structure and operation of a second exemplary embodiment of a refrigerator according to the invention,

FIG. 5 schematically shows a second example of a working cycle of the working fluid of the refrigerator in FIG. 3.

With reference to the exemplary embodiment in FIG. 1, the refrigerator according to the invention is suitable for transferring heat from a cold source 15 at a cryogenic temperature to a hot source at ambient temperature 1 for example.

The cold source 15 may, for example, be liquid nitrogen for cooling and the hot source 1 may be water or air. To carry out this heat transfer, the refrigerator shown in FIG. 1 uses a working circuit 200 of a working gas comprising the components listed below.

The circuit 200 comprises a plurality of centrifugal compressors 3, 5, 7 disposed in series and operating at ambient temperature.

The circuit 200 comprises a plurality of heat exchangers 2, 4, 6 operating at ambient temperature disposed respectively at the outlet of the compressors 3, 5, 7. The temperatures of the working gas at the inlet and outlet of each compression stage (that is at the inlet and outlet of each compressor 3, 5, 7) are kept by the heat exchangers at a substantially identical level (cf. zone A in FIG. 3 which shows a gas working cycle: temperature in K as a function of the entropy S in J/kg). In FIG. 3, the rising portions of zone A in a sawtooth pattern each correspond to a compression stage, while the descending portions of this zone A each correspond to a cooling by heat exchanger.

This arrangement serves to approach isothermal compression. The inlet and outlet temperatures of each compression stage are substantially the same.

The heat exchangers 2, 4, 6 may be different or may be composed of distinct portions of the same heat exchanger in heat exchange with the hot source 1.

The refrigerator comprises a plurality of high-speed motors (70 cf. FIG. 2). In the context of the present invention, high-speed motor normally means motors whereof the speed of rotation allows a direct coupling with a centrifugal compression stage or a centripetal expansion stage. The high-speed motors 70 preferably use magnetic or dynamic gas bearings 171 (FIG. 2). A high-speed motor typically rotates at a speed of 10 000 rpm or several tens of thousands of rpm. A low-speed motor rotates at a speed of a few thousand rpm.

Downstream of the compression portion comprising the compressors in series, the refrigerator comprises a heat exchanger 8 preferably of the countercurrent plate type separating the elements at ambient temperature (in the upper part of the circuit 200 shown in FIG. 1) from the elements at cryogenic temperature (in the lower part of the circuit 200). The fluid is cooled (corresponding to zone D in FIG. 3). The cooling of the gas from ambient temperature to cryogenic temperature takes place by countercurrent exchange with the same working gas at cryogenic temperature, which originates from the expansion portion after heat exchange with the cold source 15.

Downstream of this cooling portion comprising the plate heat exchanger 8, the circuit comprises one or more expansion turbines 9, 11, 13, preferably of the centripetal type, disposed in series. The turbines 9, 11, 13 operate at cryogenic temperature, the inlet and outlet temperatures of each expansion stage (turbine inlet and outlet) are kept substantially identical by one or more cryogenic heat exchangers 10, 12, 14 disposed at the outlet of the turbine(s). This corresponds to zone C in FIG. 3, the descending portions of zone C each corresponding to an expansion stage while the rising portions of this zone correspond to the heating in the heat exchangers 10, 12, 14. This arrangement serves to approach an isothermal expansion. The inlet and outlet temperatures of each expansion stage are substantially the same. Moreover, and in order to increase the efficiency of the refrigerator, the increase in the working gas temperature in the heat exchanger(s) (10, 12, 14) may be substantially identical (in absolute value) to the drop in the temperature of the fluid to be cooled (15) (cold source).

These heating heat exchangers 10, 12, 14 may be different or may be composed of distinct portions of the same heat exchanger exchanging heat with the cold source 15.

Downstream of the expansion and heat exchange portion with the cold source 15, the working fluid again exchanges heat with the plate heat exchanger 8 (zone B in FIG. 3). The fluid exchanges heat in the heat exchanger 8 in countercurrent to its passage after the compression portion. After heating, the fluid returns to the compression portion and can repeat its cycle.

The circuit may further comprise a chamber of working gas at ambient temperature (not shown for the sake of simplification) to limit the pressure in the circuits, during the shutdown of the refrigerator for example.

The refrigerator preferably uses as working fluid a fluid in the gas phase flowing in a closed circuit. This is composed for example of a pure gas or a mixture of pure gases. The most suitable gases for this technology are in particular: helium, neon, nitrogen, oxygen and argon. Carbon monoxide and methane may also be used.

The refrigerator is designed and thus operated so as to obtain a fluid working cycle approaching the reverse Ericsson cycle. This means: an isothermal compression, an isobaric cooling, an isothermal expansion and an isobaric heating.

According to an advantageous feature, in order to drive at least the compressors 3, 5, 7 (that is to drive the compressor impellers), the refrigerator uses a plurality of high-speed motors 70.

As shown schematically in FIG. 2, each high-speed motor 70 accommodates a compressor impeller 31 on one end of its output shaft and, on the other end of its output shaft, another compressor impeller or a turbine wheel 9. This arrangement provides many advantages. This configuration allows a direct coupling in the refrigerator between the motor 70 and the impellers of the compressor 3, 5, 7 or between the motor 70 and the wheels of the turbines 9, 11, 13. This serves to do without a speed step-up gear or reducer (thereby limiting the number of moving parts required). This configuration also allows the utilization of the mechanical work of the turbine(s) 9, 11, 13 and in consequence, an increase in the total energy efficiency of the refrigerator. According to this configuration, the refrigerator operates without oil, thereby guaranteeing the purity of the working gas and eliminating the need for a de-oiling operation.

The number of high-speed motors mainly depends on the energy efficiency desired for the refrigerator. The higher this efficiency, the higher the number of high-speed motors.

The ratio between the number of compression stages (compressors) and the number of expansion stages (turbines) depends on the target cold temperature. For example, for a refrigerator of which the cold source is at 273 K, the number of compression stages is substantially equal to the number of expansion stages. For a refrigerator in which the cold source is at 65 K, the number of compression stages is about 3 times higher than the number of expansion stages.

FIG. 4 shows another embodiment which can be used for example to cool or maintain the temperature of superconductor cables at a cryogenic temperature of about 65 K. For this temperature level, the number of compression stages (compressors) must be about three times higher than the number of expansion stages (turbines). This can be obtained in several possible configurations. For example, three compressors and one turbine or six compressors and two turbines.

The choice of the number of units depends on the desired energy efficiency. Thus, a solution using three compressors and one turbine will have a lower energy efficiency than a solution using six compressors and two turbines.

In the example in FIG. 4, the refrigerator comprises six compressors 101, 102, 103, 104, 105, 106 and two turbines 116, 111 and four high-speed motors 107, 112, 114, 109. The first two compressors 101, 102 (that is the compressor impellers) are mounted respectively at the two ends of a first high-speed motor 107. The next two compressors 103, 104 are mounted respectively on the two ends of a second high-speed motor 112. The next compressor 105 and the turbine 116 (that is the turbine wheel) are mounted respectively on the two ends of a third high-speed motor 114. Finally, the last turbine 111 and the sixth compressor 106 are mounted respectively on the two ends of a fourth motor 109.

The routing of the working gas during a cycle in the closed loop circuit can be described as follows.

In a first step, the gas is progressively compressed by passing in succession through the four compressors in series 101, 102, 103, 104, 105, 106.

On completion of each compression stage (at the outlet of each compressor) the working gas is cooled in a respective heat exchanger 108 (by heat exchange with air or water for example) to approach isothermal compression. After this compression portion, the gas is isobarically cooled through a countercurrent plate heat exchanger 103. After this cooling portion, the cooling gas is progressively expanded in the two centripetal turbines in series 116, 111. After each expansion stage the working gas is heated by heat exchange in a heat exchanger 110 (for example by heat exchange with the cold source), in order to obtain a substantially isothermal expansion. On completion of this isothermal expansion, the working gas is heated in the heat exchanges 113 and can then start a new cycle by a compression.

FIG. 5 shows the cycle (temperature T and entropy S) of the working fluid of the refrigerator in FIG. 5. As previously for FIG. 3, six sawteeth can be distinguished in the compression zone A, corresponding to the six successive compressions and coolings. In the expansion zone C, two sawteeth are identified, corresponding to the two successive expansions and heatings.

The invention improves the cryogenic refrigerators in terms of energy efficiency, reliability and size. The invention serves to decrease the number of maintenance operations and to eliminate the use of oils.

Obviously, one or both ends of the output shafts of the motor(s) can directly drive more than one wheel (that is a plurality of compressors or a plurality of turbines). 

1-10. (canceled) 11: A cryogenic refrigeration device for transferring heat from a cold source to a hot source via a working fluid flowing through a closed working circuit, the working circuit comprising in series; a) a portion for the substantially isothermal compression of the fluid, b) a portion for the substantially isobaric cooling of the fluid, c) a portion for the substantially isothermal expansion of the fluid, and d) a portion for the substantially isobaric heating of the fluid, the compression portion of the working circuit comprising; e) at least two compressors disposed in series and f) at least one heat exchanger for cooling the compressed fluid disposed at the outlet of each compressor, the expansion portion of the working circuit comprising; g) at least one expansion turbine and h) at least one heat exchanger for heating the expanded fluid, wherein the compressors and the expansion turbine(s) are driven by at least one high-speed comprising; i) an output shaft whereof one end supports and rotates, by means of direct coupling, j) a first compressor and whereof the other end supports and rotates, by means of direct coupling, k) a second compressor or an expansion turbine, and in that l) the compressors are of the centrifugal compression type, and in that m) the expansion turbine(s) are of the centripetal expansion type, and in that n) the output shafts of the motors are mounted on magnetic or dynamic gas bearings, said bearings being used to support the compressors and the turbine sand in that o) the cooling portion and the heating portion comprise a common heat exchanger through which the working fluid flows in countercurrent according to whether it is cooled or heated. 12: The device of claim 11, wherein the working circuit comprises a volume forming a buffer storage chamber for the working fluid. 13: The device of claim 11, wherein the working fluid is in the gas phase and is composed of a pure gas or a mixture of pure gases selected from the group consisting of: helium, neon, nitrogen, oxygen, argon, carbon monoxide, methane, or any other fluid having a gas phase at the temperature of the cold source. 14: The device of claim 11, wherein the number of compression stages is higher than the number of expansion stages. 15: The device of any claim 11, further comprising at least one motor whereof at least one end of the output shaft rotates, by means of direct coupling, at least two wheels. 16: The device of claim 15, further comprising at least one motor whereof one end of its output shaft rotates, by means of direct coupling, two compressor impellers, the other end of the output shaft rotating, by means of direct coupling, a turbine wheel. 17: A cryogenic refrigeration method for transferring heat from a cold source to a hot source via a working fluid flowing through a closed working circuit, the working circuit comprising in series; a) providing a compression portion comprising at least two compressors disposed in series, b) providing a fluid cooling portion, c) providing an expansion portion comprising at least one expansion turbine, and d) providing a heating portion, the method comprising a working cycle comprising; e) compressing, substantially isothermally, the fluid in the compression portion by cooling the compressed fluid at the outlet of the compressors, f) cooling, substantially isobaricly, the fluid in the cooling portion, g) expanding, substantially isothermally, the fluid in the expansion portion by heating the expanded fluid at the turbine outlet, and h) heating, substantially isobaricly, the fluid having exchanged heat with the cold source), the fluid working cycle (temperature T, entropy S) being of the reverse Ericsson type, i) cooling, during the first substantially isothermal compression step, the compressed fluid at the outlet of each compressor to keep the fluid temperatures at the inlet and outlet of each compressor substantially equal and preferably within a range of about 10 K, j) cooling, during the third substantially isothermal expansion step, the expanded fluid at the outlet of each turbine to keep the fluid temperatures at the inlet and outlet of each turbine substantially equal and preferably within a range of about 5 K, k) driving the compressors and the expansion turbine(s) by at least one high-speed motor comprising an output shaft whereof one end supports and rotates, by means of direct coupling, a first compressor and whereof the other end supports and rotates, by means of direct coupling, a second compressor or an expansion turbine, and in that l) transferring part of the mechanical work of the turbine(s) to the compressor(s) via the output shaft(s), and in that m) mounting the output shafts of the motors on magnetic or dynamic gas bearings, said bearings being used to support the compressors and turbines, and in that the cooling portion and the heating portion comprise a common heat exchanger through which the working fluid flows in countercurrent according to whether it is cooled or heated. 18: The method of claim 17, wherein on completion of the second cooling step, the working fluid is cooled to a low temperature of about 60 K and in that the working circuit comprises a number of compressors that is three times higher than the number of expansion turbines. 19: The method of claim 17, wherein the working fluid is used to cool or to keep cold superconductor elements at a temperature of about 65 K. 20: The method of claim 17 wherein the temperature drop of the fluid constituting the cold source is substantially identical to the temperature rise of the working gas in heat exchanger 